(see above for file location details etc)
Copy the following files to new folder "DOTT" in your SCUMMVM games folder:-
/PSP/GAME/SCUMM....(depends on your name for the folder!)/GAMES/DOTT
TENTACLE.000
TENTACLE.001
TENTACLE.EXE
MONSTER.SOU (or MONSTER.SO3 if its been compressed)

That's it - you just need to run scummvm and ADD the game (by navigating to the DOTT folder and clicking "CHOOSE"

FoneFreak - I know people have played it on PSP, but not sure if they compressed it - I'm pretty sure there will be a way to compress it (but by how much - who knows!) - check out some of the sites i mentioned before you might find something useful ?

Softfiz - RE grim fandango: no (well, not yet) it uses a different game engine (ie not SCUMM based). I think someone was working on an engine though (?)

I know this might be a little out there... but is there any chance of getting Grim Fandengo to run on the PSP using Scummvm?

Not a single chance. ScummVM doesn't support on any platform, so this also applies to the PSP. There is sub-project called Residual (look for Projects on scummvm.org) which attempts to "emulate" Grim Fandango, but it's still in its early stages.